The present invention relates to the orthopedic field and the provision of prostheses, such as hip and knee implants, as well as methods of manufacture of such devices and material used therein.
The use of synthetic polymers, e.g., ultra high molecular weight polyethylene, with metallic alloys has revolutionized the field of prosthetic implants, e.g., their use in total joint replacements for the hip or knee. Wear of the synthetic polymer against the metal of the articulation, however, can result in severe adverse effects which predominantly manifest after several years. Various studies have concluded that such wear can lead to the liberation of ultrafine particles of polyethylene into the periprosthetic tissues. It has been suggested that the abrasion stretches the chain folded crystallites to form anisotropic fibrillar structures at the articulating surface. The stretched-out fibrils can then rupture, leading to production of submicron sized particles. In response to the progressive ingress of these polyethylene particles between the prosthesis and bone, macrophage-induced resorption of the periprosthetic bone is initiated. The macrophage, often being unable to digest these polyethylene particles, synthesize and release large numbers of cytokines and growth factors which can ultimately result in bone resorption by osteoclasts and monocytes. This osteolysis can contribute to mechanical loosening of the prosthesis components, thereby sometimes requiring revision surgery with its concomitant problems.
It is an object of the invention to provide an implantable prosthesis device formed at least in part of melt-irradiated ultra high molecular weight polyethylene (UHMWPE) which results in reduced production of fine particles from the prosthesis during wear of the prosthesis.
It is another object of the invention to reduce osteolysis and inflammatory reactions resulting from prosthesis implants.
It is yet another object of the invention to provide a prosthesis which can remain implanted within a person for prolonged periods of time.
It is yet another object of the invention to provide improved UHMWPE which can be used in the prostheses of the preceding objects.
Still another object of the invention is to provide improved UHMWPE which has a highly entangled and low crystalline polymeric structure.
A still further object of the invention is to provide improved UHMWPE which is relatively transparent and wear resistant.
According to the invention, a medical prosthesis for use within the body, e.g., for hip or knee joint replacement, is provided. It is formed at least in part of an UHMWPE which has a polymeric structure with less than about 50% crystallinity, less than about 290 xc3x85 lamellar thickness and less than about 940 MPa tensile elastic modulus, so as to reduce production of fine particles from the prosthesis during wear of the prosthesis. In one embodiment the UHMWPE has a polymeric structure with about 40% crystallinity, about 100 xc3x85 lamellar thickness and about 200 MPa tensile elastic modulus. Preferably, the UHMWPE has substantially no trapped free radicals, has reduced hardness, is substantially not oxidized, has substantially no chain scission, has extensive cross-linking so that a substantial portion of the zpolymeric structure does not dissolve in Decalin at 150xc2x0 C. over a period of 24 hours, and/or has a high density of entanglement so as to cause the formation of imperfect crystals and reduce crystallinity. The UHMWPE initially has a molecular weight greater than about 500,000, preferably greater than about 1,000,000 and more preferably greater than about 2,000,000. In certain embodiments, part of the prosthesis is in the form of a cup shaped article having a load bearing surface made of this UHMWPE. This load bearing surface can be in contact with a second part of the prosthesis having a mating load bearing surface of a metallic or ceramic material.
Another aspect of the invention is UHMWPE having a unique polymeric structure characterized by less than about 50% crystallinity, less than about 290 xc3x85 lamellar thickness and less than about 940 MPa tensile elastic modulus. The UHMWPE can also have high transmissivity of light.
Yet other aspects of the invention are fabricated articles, e.g., with a load bearing surface, and transparent and wear resistant coatings, made from such UHMWPE. One embodiment is where the fabricated article is in the form of a bar stock which is capable of being shaped into articles by conventional methods, e.g., machining.
Yet another aspect of the invention includes a method for making highly entangled and crosslinked UHMWPE. Conventional UHMWPE, in the form of, e.g., a powder, a bar stock, a shaped bar stock, a coating or a fabricated article, e.g., a cup shaped article for use in a prosthesis, is provided. Preferably, the UHMWPE starting material has an average molecular weight of greater than about 2 million. The UHMWPE is surrounded with an inert material that is substantially free of oxygen. The UHMWPE is heated at or above its melting temperature, preferably, at about 145xc2x0 C. to about 230xc2x0 C., more preferably at about 175xc2x0 C. to about 200xc2x0 C., for a time sufficient to allow the polymer chains to achieve an entangled state, e.g., for a period of about 5 minutes to about 3 hours. The heated UHMWPE is then crosslinked so as to trap the polymer chains in the entangled state using irradiation, e.g., with gamma irradiation or electron irradiation, preferably allowing a dose greater than about 1 MRad, more preferably a dose greater than about 20 MRad. The irradiated UHMWPE is then cooled to about 250xc2x0 C., preferably at a rate equal to or greater than about 0.5xc2x0 C./min., more preferably at a rate equal to or greater than about 120xc2x0 C./min. In certain embodiments, the cooled UHMWPE is then machined or compression molded.
Yet another aspect of the invention includes the product made in accordance with this method.
The invention also features a method of making a prosthesis from UHMWPE so as to reduce production of fine particles from the prosthesis during wear of the prosthesis. UHMWPE having a polymeric structure with less than about 50% crystallinity, less than about 290 xc3x85 lamellar thickness and less than about 940 MPa tensile elastic modulus is provided. A prosthesis is formed from this UHMWPE, the UHMWPE forming a load bearing surface of the prosthesis.
Yet another aspect of the invention includes a method of treating a body in need of a prosthesis. A shaped prosthesis formed of ultra high molecular weight polyethylene having a polymeric structure with less than about 50% crystallinity, less than about 290 xc3x85 lamellar thickness and less than about 940 MPa tensile elastic modulus, is provided. This prosthesis is applied to the body in need of the prosthesis.
The above and other objects, features and advantages of the present invention will be better understood from the following specification when read in conjunction with the accompanying drawings.